A reader has a disagreement with a colleague over the construction “I don’t know, man.” The colleague insists that man should be capitalized because it’s a vocative that has the same status as Mom or Captain or a person’s name in direct address. The colleague goes so far as to say that all such substitutes for a title or name should be capitalized: Boss, Sweetheart, Dude, Brother, Toots.

My correspondent doesn’t agree but has yet to marshal a conclusive argument. Let me try to help.

First, in a general way, the tendency in British and American English over the past century has been to reduce the amount of capitalization.

Second, there is a common distinction between nicknames and mere vocatives or terms of endearment, and a quick Google search on vocative capitalization will turn up multiple sources. When you say to your father, “You’re not as funny as you think you are, Dad,” Dad is a nickname, a substitute for the person’s name. But a waitress’s “What’ll you have, hon?” is more of a generic placeholder than a nickname.

If everybody in the office calls the manager “Boss,” as in “I heard Boss say he wants your memo by five o’clock or there’ll be hell to pay,” that’s a nickname and should be capitalized. But in “It’ll be on your desk, boss,” though not exactly a term of endearment, boss is in the same class of lowercased terms of address as sweetheart, dear, and the like.

The colleague’s insistence on capitalization in all instances flies in the face of common practice.